Add Global Profile API support

[tholovina-sift]

Purpose:

• Add client support for the new Global Profile API, which returns cross-tenant identity, decision, chargeback, order, transaction, and location signals for a user
• Expose two new endpoints: GET /v3/accounts/{accountId}/global_profile/users/{userId} (lookup by Sift user ID) and POST /v3/accounts/{accountId}/global_profile/lookup (lookup by email and/or phone)

Technical overview:

• GlobalProfileRequest — GET endpoint; takes AccountId, UserId, and optional GlobalOnly (bool?) and IncludeOwnData (bool?) query params; params are only added to the URL when explicitly set so server-side defaults apply otherwise
• GlobalProfileLookupRequest — POST endpoint; takes AccountId, Email, and Phone; throws MissingFieldException client-side before the request is built when both Email and Phone are missing, mirroring the existing required-field validation pattern
• GlobalProfileResponse : SiftResponse with nested POCOs for all response fields (ProfileSummaryJson, IdentityAgeJson, UserDecisionsJson, ChargebacksJson, OrdersJson, TransactionsJson, LocationsJson, LocationConnectedAccountJson); all nested objects and count fields are nullable so identity_found: false responses deserialize cleanly
• Client gains SendAsync(GlobalProfileRequest) and SendAsync(GlobalProfileLookupRequest) overloads following the existing SendAsync<T> dispatch pattern

Testing plan:

• This PR contains all required labels and reviewers as per our Change Management Procedure
• Live tested against the Sift API with a real account: both GET and POST returned status=0, error_message=OK with identity_found=True, link_count=31, and fully populated location entries including city, region, and country

Deployment plan:

• Related documentation is updated

  • README.md: new "Global Profile" section with usage examples for both endpoints; version heading bumped to v1.8.0
  • CHANGES.MD: new 1.8.0 entry
  • Sift/Sift.csproj: version bumped to 1.8.0

• Merge when Global Profile API is publicly available

• Merge before https://github.com/SiftScience/sift-docs/pull/92

Rollback plan:

• Revert this PR; no schema changes, no breaking changes to existing methods, fully additive

Customer Impact Assessment:

• as per our Change Management Procedure